Tabular silver halide grains, their preparation and use in photographic emulsions are widely known. They have been extensively studied in the literature since photographic emulsions containing these grains appeared to offer some significant advantages over photographic emulsions containing round or globular grains (e.g., splash prepared types). Generally, tabular grains are large, flat silver halide grains that are prepared by employing long ripening times or by balanced double jet (BDJ) accelerated flow precipitation methods. Commercial emulsions using tabular grains are conventionally made by using a BDJ process. The tabular grains usually have triangular or hexagonal parallel crystal faces, each of which is usually larger than any other crystal face of the grain and are conventionally defined by their aspect ratio (AR) which is the ratio of the diameter of the grain to the thickness. Tabular grains of varying thicknesses and AR's have been found to be useful in photographic systems. Large diameter high aspect ratio grains, e.g., at least 8:1, have diameters of at least 0.6 .mu.m and thicknesses of less than 0.3 .mu.m. These larger tabular grains have certain commercial advantages now apparent to those of normal skill in the art. For example, they have a larger surface area and, thus, can accept more sensitizing dye. Since these tabular grains usually are dye sensitized when emulsions using such tabular grains are present in medical x-ray elements, an increase in sharpness can result. In addition, since the tabular grains normally lie flat when coated from an emulsion on a support, the covering power is usually greater and, thus, the emulsions can be coated at a lower coating weight and are, therefore, less costly.
The tabular grains have found wide use in full color, black-and-white, and x-ray films. However, in order to be successfully used in x-ray films, it is necessary that the tabular grain containing x-ray films have the same image tone as prior x-ray films. This is necessary as radiologists are used to this tone and demand that the tone remain the same in order that they may successfully read x-ray photographs in the same way they have in the past. In order to form T-grains satisfactory for x-ray films, it is necessary that they have a certain thickness and equivalent circular diameter range. The forming of grains within the preferred range has required long formation times, as well as the use of difficult to control materials. Nottorf U.S. Pat. No. 4,722,886 discloses the formation of silver halide materials suitable for use in x-ray film. As disclosed there, the process is performed with the pH of the digestion solution raised by the addition of ammonium hydroxide as a solvent. However, the use of ammonium hydroxide as a solvent sometimes lead the R-typing problem causing fog. Additionally, the use of ammonium hydroxide is difficult to control as the material is volatile and evaporates. Further, use of ammonium hydroxide requires a high amount of addition to raise the pH and results in problems with a effluent discharge as ammonium hydroxide is difficult to dispose of without special treatment.
There remains a need for a method of forming tabular silver halide emulsions at increased speed without problems of volatility of the base or difficulties in waste water disposal.